Explosion-proof nonaqueous electrolyte secondary cell and rupture pressure setting method therefor

ABSTRACT

An explosion-proof non-aqueous electrolyte battery is provided which has a current cutoff device for cutting off electrical connection within the battery with safety and certainty in the event of overcharge while remaining inoperable in normal use or in storage at an elevated temperature. A pair of an upper vent plate  1  and a lower vent plate  2  mechanically and electrically coupled with each other is provided on the sealing part of-the battery where the mechanical coupling of the pair of the upper vent plate  1  and the lower vent plate  2  is so configured as to rupture and cut off an electric current in the event the internal pressure of the battery case increases beyond a predetermined value, and the rupture pressure at which the mechanical coupling between the pair of upper vent plate  1  and the lower vent plate  2  is broken has been set to decrease as the spatial volume occupancy ratio of the battery increases.

This application is a U.S. National Phase Application of PCTInternational Application PCT/JP98/05051.

FIELD OF THE INVENTON

The present invention relates to an explosion-proof non-aqueouselectrolyte secondary battery adaptable to lithium secondary batteriesand the like.

BACKGROUND OF THE INVENTION

In recent years there has been a rapid progress in portable and cordlessdesigns of electronic equipment such as audio-visual equipment andpersonal computers. As the power source of these equipment, non-aqueouselectrolyte (organic solvent type liquid electrolyte) secondarybatteries as represented by various types of high-capacity alkalinestorage batteries and lithium-ion secondary batteries are suitable.Furthermore, as a result of an effort of developing sealed type versionswith a high energy density and superior load characteristic, sealed typenon-aqueous electrolyte secondary batteries are in wide use as the powersource of portable equipment including watches and cameras.

Now, with the non-aqueous electrolyte secondary battery, a chemicalchange in the power generating elements inside the battery takes placein the event of failure of applied equipment including the charger,overcharge, or misuse. For instance, an abnormal reaction due toovercharge or short circuit decomposes the electrolyte or activematerials, thus causing unusual gas evolution inside the battery and anexcessive internal pressure of the battery. For this reason this type ofbatteries has to date been provided with the following explosion-proofmechanism. That is, in the event the battery internal pressure increasesbeyond a designed value, a vent member that has been exerted with theinternal pressure is pressed toward the direction of the internalpressure (the direction of diffusion of the internal pressure) anddeformed, thereby causing rupture of a thin portion of electricallyconducting member or separation of the weld between the vent member andthe electrically conducting member thus cutting off the electric currentin the initial stage of occurrence of overcharge or short circuit andstopping the abnormal reaction. As a result, an increase in the batterytemperature or battery internal pressure due to charging current orshort-circuit current can be controlled and safety of the battery issecured.

The battery structure disclosed in U.S. Pat. No. 4,943,497 is valuablein that it provides a commercially useful product which is fullyprotected internally against overcharge. The gas evolution mechanismwhich activates the cutoff device depends on the voltage and temperatureof the battery and on time. The speed of gas evolution is not constant;it increases with increasing voltage and temperature of the battery. Itis of special significance to note in actual use of the battery thatevolution of a gas continues with time at predetermined voltage andtemperature. While it is required that evolution of a gas duringovercharge assures safe stoppage of battery operation, sustainedevolution of the gas in normal operation must be avoided. Unless gasevolution can be avoided, decomposition product of the gas willaccumulate with time causing an increase in the gas pressure presentinga possibility of operation of the cutoff device in normal use.

By a proper selection of the liquid electrolyte and positive activematerial, safety requirements can actually be satisfied to a certaindegree. However, these requirements hinder the selection because of suchother reasons as cost, complexity, and energy density. Furthermore, whenthe gas evolution continues in a completely sealed battery, the maximumlife of the cutoff device will be limited.

The prime object of the present invention is to provide a safe andexplosion-proof non-aqueous electrolyte secondary battery which isequipped with a cutoff device for cutting off electrical connectionwithin the battery with safety and certainty in the event of overchargewhile remaining inoperable in normal use or storage at an elevatedtemperature, and which is free from the danger of explosion bycombination and optimization of safety devices and superior in safety inthe event of evolution of a gas in excess of the gas discharge capacityas a result of abandonment or throwing into fire of the battery.

SUMMARY OF THE INVENTION

In order to achieve the above object, the present invention provides astructure in which a sealing part includes a pair of upper and lowervent plates mechanically and electrically coupled with each other, themechanical coupling between the pair of upper and lower vent platesbeing broken in the event the internal pressure of the battery caseincreases beyond a predetermined value thus cutting off the electriccurrent, and, in the event the internal pressure of the battery casefurther increases beyond a predetermined value an easy-to-break portionformed on the upper vent plate ruptures so that the gas inside thebattery case be released to outside. Here the rupture pressure at whichthe mechanical coupling between the pair of upper and lower vent platesis broken is set in a manner that it decreases as the spatial volumeoccupancy ratio of the battery increases, and the rupture pressure ofthe easy-to-rupture portion of the upper vent plate is set at a value inthe range 18-24 kgf/cm².

Furthermore, the present invention provides a structure in which asealing part includes a pair of upper and lower vent plates mechanicallyand electrically coupled with each other, the mechanical couplingbetween the pair of upper and lower vent plates being broken in theevent the internal pressure of the battery case increases beyond apredetermined value thus cutting off the electric current, and, in theevent the internal pressure of the battery case further increases beyonda predetermined value an easy-to-break portion formed on the upper ventplate rupturingso that-theg the battery case be released to outside.Also an easy-to-rupture portion is formed on the bottom surface of thebattery case the rupture pressure of which being set higher than therupture pressure of the easy-to-rupture portion of the upper vent plateby at least 16 kgf/cm², and the caulking withstand pressure of thesealing part is set higher than the upper limit of the rupture pressureof the easy-to-rupture portion formed on the bottom surface of thebattery case by at least 10 kgf/cm².

According to the above described invention, a safe and explosioin-proofnon-aqueous secondary battery is provided which is equipped with acutoff device for cutting off electrical connection within the batterywith safety and certainty in the event of overcharge while remaininginoperable in normal use or storage at an elevated temperature, andwhich is free from the danger of explosion by combination andoptimization of safety devices and superior in safety in the event ofevolution of a gas in excess of the gas discharge capacity as a resultof abandonment or throwing into fire of the battery.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a vertical cross-sectional view of an explosion-proof batteryin an exemplary embodiment of the present invention.

FIG. 2 is a bottom view of the battery.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of the present invention are described in thefollowing with reference to drawings. FIG. 1 is a verticalcross-sectional view of an explosion-proof lithium secondary battery inan exemplary embodiment of the present invention. In the figure, theexplosion-proof seal plate employed in the battery includes an uppervent member (upper vent plate) 1 composed of a thin metal foil, a lowervent member (lower vent plate) 2 composed of a thin metal foil anddisposed opposite to the upper vent member 1, a ring-shaped insulatinginner gasket 3 interposed between the peripheries of each of the uppervent member 1 and the lower vent member 2, a ring-shaped PTC device 4placed on top of the periphery of the upper vent member 1, a metal cap 5laid on top of the PTC device 4 and having 6 discharge holes 5 a, and ametal case 6 made of aluminum for receiving and securing each of theabove members under a stacked condition and having 4 ventilation holes.

The inner gasket 3 is made of synthetic resin having resistance toliquid electrolyte, for example silane-cross-linked polypropylene typepolymer, formed in a manner that a tubular portion 3 b extends upwardlyfrom the outer edge of a ring-shaped periphery 3 a. The upper ventmember 1 is composed of an aluminum disk, 0.15 mm in thickness and 12.7mm in outer diameter, for example, and has a central concave portion 1 adownwardly swelling into a curvature surface, and an easy-to-rupturethin annular C-letter shape portion 1 b formed on the circumference ofthe concave portion 1 a by using a C-letter shape stamp.

The lower vent member 2 is composed of an aluminum disk 0.1 mm inthickness and 13.5 mm in outer diameter, for example, and has aneasy-to-rupture thin portion 2 b formed on the circumference of thecentral portion 2 a by using an annular stamp. The thin portion 2 b hasa rupture strength such that it ruptures when the battery internalpressure reaches a predetermined value, and the rupture strength is setat a value lower than the rupture strength of the thin portion 1 b ofthe upper vent member 1.

The central portions of each of the upper vent member 1 and the lowervent member 2 are welded to form a bonded part S, and the upper ventmember 1 and the lower vent member 2 are electricafly connected onlythrough the bonded part S. The PTC device 4 is a positive temperaturecoefficient resistance element the electrical resistance of whichdrastically increases when a predetermined temperature range is exceededby passage of an electric current in excess of a predetermined value.

Next, a brief description of a battery obtained by sealing the openingof the battery case 7 with the above described explosion-proof sealplate will be given. First, in inserting the explosion-proof seal plateinto the battery case 7 having a thin portion 7 b on the bottom, a leadmember 8 taken out from one of the electrodes (usually positiveelectrode) of an electrode group 10 housed inside the battery case 7 isconnected by welding with a battery case 6, followed by pouring liquidelectrolyte in the electrode group 10 and fitting the explosion-proofseal plate to inside of the opening of the battery case 7 withintervention of an insulating gasket 9 on the circumference.Subsequently, the peripheral edge portion 7 a of the opening of thebattery case 7 is caulked inward thus sealing the battery case 7 withthe explosion-proof seal plate. In the meantime, the thin portion 7 b prC-letter shape and forms an easy-to-rupture portion.

The electric current of this battery flows from the electrode group 10housed inside the battery case 7 through the lead member 8, metal case6, lower vent member 2, bonded part S, upper vent member 1, PTC device4, to the metal cap 5 which also serves as an external terminal therebyenabling the battery to function. Here, in the event an excessivecurrent flows, the PTC device 4 reaches at the operating temperature ina short period of time and increases its resistance thus greatlyreducing the current and maintaining it at a low level. As a resultsevere damage of the battery due to external short circuit or misuse atan excessively large current may be prevented.

In the event of overcharge or reverse charge due to failure of thecharger, or overdischarge of many batteries connected in series, itoften happens that the current exceeds the allowable safety limit eventhough it is below the operating current of the PTC device 4 and thebattery internal pressure increases. When the battery internal pressurereaches a predetermined value established based on the rupture strengthof the thin portion 2 b of the lower vent member 2, a part of the thinportion 2 b of the lower vent member 2 which receives the batteryinternal pressure through a ventilation (breathing) hole 6 a ruptures,and by the pressure received through the ruptured part the upper ventmember 1 is deformed upward by the flip of the concave portion 1 atriggering rupture of the thin portion 2 b of the lower vent member 2 byshearing. As a result, the portion encircled by the thin portion 2 b ofthe lower vent member 2 is removed away from the lower vent member 2together with the upper vent member 1, thus the upper vent member 1 andthe lower vent member 2 which have been electrically conducting onlythrough the bonded part S are separated, thus cutting off the electriccurrent.

In the event the battery internal pressure further continues to increasedue to evolution of a large volume of gas and the battery internalpressure reaches a predetermined value established based on the rupturestrength of the thin portion 1 b of the upper vent member 1, the thinportion 1 b ruptures open and the filled gas is discharged to outside ofthe battery through the discharge holes 5 a.

In the event the battery internal pressure continues to rapidly increaseand the battery internal pressure reaches the rupture strength of thethin portion 7 b of the bottom of the battery case 7, the thin portion 7b ruptures open and the filled gas is discharged to outside of thebattery.

Example cylindrical batteries A1 to A7 having the above structure werefabricated as follows. A battery having a spatial volume occupancyratio, being volume ratio of the empty space in the total space occupiedby the battery (namely, the space not constituting a power generationelement), of 15-20% and using a seal plate having a rupture pressure ofthe lower vent member 2 set at 3-11 kgf/cm² is designated A1, a batteryhaving a spatial volume occupancy ratio of 10-14% and using a seal platehaving a rupture pressure of the lower vent member 2 set at 4-13 kgf/cm²is designated A2, a battery having a spatial volume occupancy ratio of9% and using a seal plate having a rupture pressure of the lower ventmember 2 set at 5-13 kgf/cm² is designated A3, a battery having aspatial volume occupancy ratio of 8% and using a seal plate having arupture pressure of the lower vent member 2 set at 5.5-13 kgf/cm² isdesignated A4, a battery having a spatial volume occupancy ratio of 7%and using a seal plate having a rupture pressure of the lower ventmember 2 set at 7-13 kgf/cm² is designated A5, a battery having aspatial volume occupancy ratio of 6% and using a seal plate having arupture pressure of the lower vent member 2 set at 9-14 kgf/cm² isdesignated A6, and a battery having a spatial volume occupancy ratio of5% and using a seal plate having a rupture pressure of the lower ventmember 2 set at 13-15 kgf/cm² is designated A7.

Also, example rectangular batteries F1 to F5 were fabricated by settingthe rupture pressure of the lower vent member 2 as follows. A batteryhaving a spatial volume occupancy ratio of 15-20% and using a seal platehaving a rupture pressure of the lower vent member 2 set at 2-5 kgf/cm²is designated F1, a battery having a spatial volume occupancy ratio of10-14% and using a seal plate having a rupture pressure of the lowervent member 2 set at 3-7 kgf/cm² is designated F2, a battery having aspatial volume occupancy ratio of 7-9% and using a seal plate having arupture pressure of the lower vent member 2 set at 4-7 kgf/cm² isdesignated F3, a battery having a spatial volume occupancy ratio of 6%and using a seal plate having a rupture pressure of the lower ventmember 2 set at 4-9 kgf/cm² is designated F4, and a battery having aspatial volume occupancy ratio of 5% and using a seal plate having arupture pressure of the lower vent member 2 set at 6-10 kgf/cm² isdesignated F5. Here, the rupture pressure (operating pressure) of thethin portion is measured by gas pressurization.

Results of overcharge test, elevated-temperature storage test, and firethrow in test of these batteries are described in the following.

(1) Overcharge Test:

The overcharge test is a test assuming overcharge under an uncontrolledcondition such as due to failure of the charger. The status of each ofthe batteries after the test was observed. As comparative examples ofcylindrical batteries, a battery having a spatial volume occupancy ratioof 15-20% and using a seal plate having a rupture pressure of the lowervent member 2 set at 13 kgf/cm² is designated B1, a battery having aspatial volume occupancy ratio of 10-14% and using a seal plate having arupture pressure of the lower vent member 2 set at 14 kgf/cm² isdesignated B2, a battery having a spatial volume occupancy ratio of 6%and using a seal plate having a rupture pressure of the lower ventmember 2 set at 15 kgf/cm² is designated B3, and a battery having aspatial volume occupancy ratio of 5% and using a seal plate having arupture pressure of the lower vent member 2 set at 16 kgf/cm² isdesignated B4.

On the other hand, as comparative examples of rectangular batteries, abattery having a spatial volume occupancy ratio of 15-20% and using aseal plate having a rupture pressure of the lower vent member 2 set at 6kgf/cm² is designated G1; a battery having a spatial volume occupancyratio of 7-14% and using a seal plate having a rupture pressure of thelower vent member 2 set at 8 kgf/cm² is designated G2, a battery havinga spatial volume occupancy ratio of 6% and using a seal plate having arupture pressure of the lower vent member 2 set at 10 kgf/cm² isdesignated G4, and a battery having a spatial volume occupancy ratio of5% and using a seal plate having a rupture pressure of the lower ventmember 2 set at 11 kgf/cm² is designated G5. Table 1 shows results ofovercharge test of the example batteries of the present invention andcomparative examples.

TABLE 1 Spatial volume Lower vent member occupancy ratio rupturepressure Overcharge Battery type (%) (kgf/cm²) ignition ratioCylindrical batteries Examples A1 15-20  3-11 0/100 A2 10-14  4-13 0/100A3 9  5-13 0/100 A4 8 5.5-13  0/100 A5 7  7-13 0/100 A6 6  9-14 0/100 A75 13-15 0/100 Comparative B1 15-20 13 97/100  examples B2 10-14 1493/100  B3 6 15 94/100  B4 5 16 96/100  Rectangular batteries ExamplesF1 15-20 2-5  0/100 F2 10-14 3-7  0/100 F3 7-9 4-7  0/100 F4 6 4-9 0/100 F5 5  6-10  0/100 Comparative G1 15-20  6 97/100 examples G2 7-14  8 95/100 G4 6 10 92/100 G5 5 11 94/100

From the results of Table 1, it can be seen that while the comparativeexample cylindrical batteries B1-B4 and rectangular batteries G1-G5 hadignited, none of the cylindrical batteries A1-A7 and rectangularbatteries F1-F5 of examples of the present invention have reached thestate of igniting. With the comparative example batteries B1-B4 andG1-G5, the timing of cutting off the electrical connection for anelectric current was delayed as the rupture pressure of the lower ventmember 2 was high, and decomposition of the electrolyte continued,resulting in an increase in the battery internal pressure and continuedrise in the battery temperature. The battery thermal runaway starttemperature was eventually reached thus releasing oxygen-containing gasand ending in ignition.

On the other hand, with each of the example batteries A1-A7 and F1-F5 ofthe present invention, an optimum rupture pressure for each respectivespatial volume occupancy ratio has been set and the timing of cuttingoff electrical connection for an electric current was never delayed.Consequently, the thermal runaway temperature was never reached due tocontinued rises in the battery internal pressure and temperature thusnever resulting in ignition.

(2) Elevated-Temperature Storage Test:

The elevated-temperature storage test is a test to store a battery in a85° C. thermostat oven for 3 days. Status of the battery after the testis observed. As comparative examples of cylindrical batteries, a batteryhaving a spatial volume occupancy ratio of 15-20% and using a seal platehaving a rupture pressure of the lower vent member 2 set at 2 kgf/cm² isdesignated C1, a battery having a spatial volume occupancy ratio of10-14% and using a seal plate having a rupture pressure of the lowervent member 2 set at 3 kgf/cm² is designated C2, a battery having aspatial volume occupancy ratio of 9% and using a seal plate having arupture pressure of the lower vent member 2 set at 4 kgf/cm² isdesignated C3, a battery having a spatial volume occupancy ratio of 8%and using a seal plate having a rupture pressure of the lower ventmember 2 set at 5 kgf/cm² is designated C4, a battery having a spatialvolume occupancy ratio of 7% and using a seal plate having a rupturepressure of the lower vent member 2 set at 6 kgf/cm² is designated C5, abattery having a spatial volume occupancy ratio of 6% and using a sealplate having a rupture pressure of the lower vent member 2 set at 8kgf/cm² is designated C6, and a battery having a spatial volumeoccupancy ratio of 5% and using a seal plate having a rupture pressureof the lower vent member 2 set at 12 kgf/cm² is designated C7.

Furthermore, as comparative examples of rectangular batteries, a batteryhaving a spatial volume occupancy ratio of 15-20% and using a seal platehaving a rupture pressure of the lower vent member 2 set at 1 kgf/cm² isdesignated H1, a battery having a spatial volume occupancy ratio of7-14% and using a seal plate having a rupture pressure of the lower ventmember 2 set at 2 kgf/cm² is designated H2, a battery having a spatialvolume occupancy ratio of 6% and using a seal plate having a rupturepressure of the lower vent member 2 set at 3 kgf/cm² is designated H4,and a battery having a spatial volume occupancy ratio of 5% and using aseal plate having a rupture pressure of the lower vent member 2 set at 5kgf/cm² is designated H5. Table 2 shows results of elevated-temperaturestorage test of example batteries of the present invention andcomparative example batteries.

TABLE 2 Spatial volume Lower vent member Ratio of occupancy ratiorupture pressure erroneous Battery types (%) (kgf/cm²) current cutoffCylindrical batteries Examples A1 15-20  3-11  0/100 A2 10-14  4-13 0/100 A3 9  5-13  0/100 A4 8 5.5-13   0/100 A5 7  7-13  0/100 A6 6 9-14  0/100 A7 5 13-15  0/100 Comparative C1 15-20 2 96/100 examples C210-14 3 92/100 C3 9 4 98/100 C4 8 5 93/100 C5 7 6 94/100 C6 6 8 97/100C7 5 12  93/100 Rectangular batteries Examples F1 15-20 2-5  0/100 F210-14 3-7  0/100 F3 7-9 4-7  0/100 F4 6 4-9  0/100 F5 5  6-10  0/100Comparative H1 15-20 1 92/100 examples H2  7-14 2 95/100 H4 6 3 97/100H5 5 5 94/100

It can be seen from Table 2 that, while erroneous current cutoffoperation of seal plates took place with the comparative examplebatteries C1-C7 and H1-H5 thus cutting off electrical conduction in thebatteries and not performing as batteries, no erroneous current cutoffoperation of seal plates took place with the example batteries A1-A7 andF1-F5 of the present invention. In this test, as a result of exposingbatteries to an elevated temperature of 85° C., the internal pressurerose due to evaporation of the liquid electrolyte and volume expansionof the evolving gas. With the comparative example batteries C1-C7 andH1-H5, electrical connection for the electric current was erroneouslycutoff because the rupture pressure of the lower vent member 2 of theseal plate was low. On the other hand, with the example batteries A1-A7and F1-F5 of the present invention, electrical connection for theelectric current was not erroneously cutoff because the rupture pressurehad been set at an optimum value relative to each respective spatialvolume occupancy ratio. From the above results, it can be seen that,with batteries that are provided with an explosion-proof seal platewhich has been set to an optimum rupture pressure relative to eachrespective spatial volume occupancy ratio, while the cutoff deviceoperates before ignition of the battery in the event of excessiveovercharge, the cutoff device does not operate while being stored at anelevated temperature, thus suggesting that batteries safer than;conventional batteries and with a high reliability can be fabricated.

(3) Fire Throw-in Test:

The fire throw-in test is a test assuming incineration after disposal ofbatteries, where a battery is burnt in a combustion furnace usingcharcoal or wood. Status of the battery after the test is observed. Inthe example batteries A1-A7 of the present invention, the rupturepressure of the upper vent member 1, the rupture pressure of the thinportion of the bottom of the case, and the caulking withstand pressureof the sealing part of the seal plate were respectively set at 18-24kgf/cm², 40-60 kgf/cm², and 70-90 kgf/cm². As comparative examples,batteries D1-D7 were fabricated of which the spatial volume occupancyratio and the rupture pressure of the lower vent member 2 were the sameas those of example batteries A1-A7, the rupture pressure of the uppervent member 1 of the seal plate had been set at 18-24 kgf/cm², therupture pressure of the thin portion of the bottom of the case had beenset at 17 kgf/cm², and the caulking withstand pressure of the sealingpart of the seal plate had been set at 70-90 kgf/cm²; comparativebatteries E1-E7 were fabricated of which the rupture pressure of theupper vent member 1 had been set at 18-24 kgf/cm², the rupture pressureof the thin portion of the bottom of the case had been set at 40-60kgf/cm², and the caulking withstand pressure of the sealing part of theseal plate had been set at 30 kgf/cm². Table 3 shows the results of firethrow-in test of the example batteries of the present invention and thecomparative example batteries.

TABLE 3 Upper Rupture Caulking vent member pressure of withstand Firerupture case bottom pressure of throw-in pressure thin portion sealingpart explosion Battery type (kgf/cm²) (kgf/cm²) (kgf/cm²) ratio Ex-A1-A7 18-24 40-60 70-90  0/100 amples Com- D1-D7 18-24 17 70-90  0/100parative E1-E7 18-24 40-60 30 100/100 ex- amples (Note) Spatial volumeoccupancy ratio and lower vent member rupture pressure of D1-D7 andE1-E7 are the same as those of A1-A7

It can be seen from Table 3 that, though the comparative examplebatteries D1-D7 did not explode, as the rupture pressure of the thinportion of the bottom of the battery case was lower than the rupturepressure of the upper vent member 1, in the event of a slight increasein the battery internal pressure owing to temperature rise of thebattery due to overcharge and other reasons, there occurred an instancein which the thin portion of the bottom of a battery ruptured even inthe case safety could be secured by the rupture of the upper vent member1 only of the seal plate. This may cause leakage of the liquidelectrolyte leading to failure of an equipment. Accordingly, a test wasconducted on several values of the difference between the rupturepressure of the upper vent member 1 and the rupture pressure of the casebottom thin portion. As a result, it was found preferable that therupture pressure of the case bottom thin portion be set higher than theupper limit of the rupture pressure of the upper vent member 1 by atleast 16 kgf/cm².

On the contrary, it is to be noted that comparative batteries E1-E7exploded. This is because when a battery was thrown into fire, rapidchemical reaction took place inside the battery accompanying suddenevolution of a gas inside the battery owing to unusual heating. As aresult of evolution of a gas in excess of the gas discharge capacitythrough the rupture of the upper vent member 1 disposed on the sealplate, the battery internal pressure suddenly increased beyond thecaulking withstand pressure of the sealing part thus leading to anexplosion. Consequently, a test was conducted by setting differentvalues as the difference between the rupture pressure of the case bottomthin portion and the caulking withstand pressure of the sealing part,from which it was found preferable to set the rupture pressure of thecase bottom thin portion lower than the lower limit of the caulkingwithstand pressure of the sealing part by substantially 10 kgf/cm².

On the other hand, the example batteries A1-A7 of the present inventiondid not reach the state of exploding. It is conceivable that when thebattery internal pressure increased by being thrown into fire, gasdischarge took place through rupture of the upper vent member 1 beforethe caulking withstand pressure of the sealing part had been reached,and in the event a volume of gas in excess of the gas discharge capacityevolved, gas discharge was performed through rupture of the case bottomthin portion, suggesting that proper setting of the operating pressurelevels controlled the sequence of gas discharge and allowed the evolvedgas inside the battery to smoothly discharge to outside of the battery.

In the above exemplary embodiment, although the structure is such thatan electric current is cutoff by rupture of the easy-to-rupture portion(thin portion) 2 b of the lower vent member 2 in the event the internalpressure of the battery case 7 exceeds a predetermined value, thestructure may be configured such that the easy-to-rupture portion 2 b isnot formed on the lower vent member 2 and the bonded part S rupturesthus cutting off the current. Also, the easy-to-rupture portion providedon the upper vent member 1 and the easy-to-rupture portion provided onthe bottom of the battery case can be configured not only by theC-letter shaped thin portions 1 b and 7 b as shown in the exemplaryembodiment but also by various other configurations.

INDUSTRIAL APPLICATION

The present invention provides a safe and explosion-proof non-aqueoussecondary battery which is equipped with an electric current cutoffdevice for cutting off electrical connection within the battery withsafety and certainty in the event of overcharge while remaininginoperable in normal use or storage at an elevated temperature, andwhich is free from the danger of explosion and superior in safety bycombination and optimization of explosion-proof devices in the event ofevolution of a gas in excess of the gas discharge capacity under a widerange of conditions including bandonment and throwing into fire of thebattery.

What is claimed:
 1. A method of setting rupture pressure of anexplosion-proof non-aqueous secondary battery including a sealing partprovided with a pair of upper and lower vent plates mechanically andelectrically coupled with each other, wherein the mechanical couplingbetween said pair of upper and lower vent plates ruptures thus cuttingoff an electric current in the event the internal pressure of a batterycase increased beyond a predetermined value, and in the event theinternal pressure of the battery case subsequently further increasedbeyond a predetermined value, an easy-to-rupture portion formed on theupper vent plate ruptures thus releasing the gas inside the battery caseto outside, and the rupture pressure at which the mechanical couplingbetween said pair of upper and lower vent plates ruptures has been setto decrease as the sspatial volume occupancy ratio of the batteryincreases.
 2. The method of setting rupture pressure of anexplosion-proof non-aqueous electrolyte secondary battery of claim 1,wherein the relationship between the spatial volume occupancy ratio ofthe battery and the rupture pressure of the lower vent plate at whichthe mechanical coupling between the pair of the upper and lower ventplates is broken has been set as shown in the following: Spatial volumeRupture pressure occupancy ratio (kgf/cm², figures rounded off) (%,figures rounded off) Cylindrical battery Rectangular battery 15-20  3-112-5 10-14  4-13 3-7 9  5-13 4-7 8 5.5-13  4-7 7  7-13 4-7 6  9-14 4-9 513-15   6-10.


3. The method of setting rupture pressure of an explosion-proofnon-aqueous electrolyte secondary battery of claim 1, wherein therupture pressure of the easy-to-rupture portion upper vent plate hasbeen set in the range of 18-24 kgf/cm².
 4. An explosion-proofnon-aqueous electrolyte secondary battery including a sealing partprovided with a pair of an upper and lower vent plates mechanically andelectrically coupled with each other, wherein the mechanical couplingbetween the pair of upper and lower vent plates ruptures thus cuttingoff an electric current in the event the internal pressure of thebattery case increased beyond a predetermined value, and in the eventthe internal pressure of the battery case substantially furtherincreased beyond a predetermined value an easy-to-rupture portion formedon the upper vent plate ruptures thus releasing the gas inside thebattery case to outside, and the rupture pressure has been set accordingto the method of setting rupture pressure as claimed in claim
 1. 5. Anexplosion-proof non-aqueous electrolyte secondary battery including asealing part provided with a pair of an upper and lower vent platesmechanically and electrically coupled with each other, wherein themechanical coupling between the pair of upper and lower vent platesruptures thus cutting off the electric current in the event the internalpressure of the battery case increased beyond a predetermined value, andin the event the internal pressure of the battery case subsequentlyfurther increased beyond a predetermined value an easy-to-ruptureportion formed on the upper vent plate ruptures thus releasing the gasinside the battery case to outside, and an easy-to-rupture portion isformed on the bottom surface of the of which being set at least 16kgf/cm² higher than the upper limit of the rupture pressure of theeasy-to-rupture portion of the upper vent plate.
 6. The explosion-proofnon-aqueous electrolyte secondary battery of claim 5, wherein therupture pressure of the easy-to-rupture portion formed on the bottomsurface of the battery case is in the range 40-60 kgf/cm².
 7. Theexplosion-proof non-aqueous electrolyte secondary battery of claim 5,wherein the caulking withstand pressure of the sealing part has been setat least 10 kgf/cm² higher than the upper limit of the rupture pressureof the easy-to-rupture portion formed on the bottom surface of thebattery case.
 8. The explosion-proof non-aqueous electrolyte secondarybattery of claim 7, wherein the caulking withstand pressure of thesealing part is in the range 70-90 kgf/cm².
 9. The explosion-proofnon-aqueous electrolyte secondary battery of claim 4, wherein aneasy-to-rupture portion is formed on the bottom surface of the batterycase and the rupture pressure of said easy-to-rupture portion has beenset at least 16 kgf/cm² higher than the upper limit of the rupturepressure of the easy-to-rupture portion of the upper vent plate.
 10. Theexplosion-proof non-aqueous electrolyte secondary battery of claim 9,wherein the rupture pressure of the easy-to-rupture portion formed onthe bottom surface of the battery case is in the range 40-60 kgf/cm².11. The explosion-proof non-aqueous electrolyte secondary battery ofclaim 9, wherein the caulking withstand pressure of the sealing part hasbeen set 10 kgf/cm² higher than the upper limit of the rupture pressureof the easy-to-rupture portion formed on the bottom surface of thebattery case.
 12. The explosion-proof non-aqueous electrolyte secondarybattery of claim 11, wherein the caulking withstand pressure of thesealing part is in the range 70-90 kgf/cm².
 13. The method of settingrupture pressure of an explosion-proof non-aqueous electrolyte secondarybattery of claim 2, wherein the rupture pressure of the easy-to-ruptureportion of the upper vent plate has been set in the range of 18-24kgf/cm².
 14. An explosion-proof non-aqueous electrolyte secondarybattery including a sealing part provided with a pair of an upper andlower vent plates mechanically and electrically coupled with each other,wherein the mechanical coupling between the pair of upper and lower ventplates ruptures thus cutting off an electric current in the event theinternal pressure of the battery case increased beyond a predeterminedvalue, and in the event the internal pressure of the battery casesubstantially further increased beyond a predetermined value aneasy-to-rupture portion formed on the upper vent plate ruptures thusreleasing the gas inside the battery case to outside, and the rupturepressure has been set according to the method of setting rupturepressure as claimed in claim
 2. 15. An explosion-proof non-aqueouselectrolyte secondary battery including a sealing part provided with apair of an upper and lower vent plates mechanically and electricallycoupled with each other, wherein the mechanical coupling between thepair of upper and lower vent plates ruptures thus cutting off anelectric current in the event the internal pressure of the battery caseincreased beyond a predetermined value, and in the event the internalpressure of the battery case substantially further increased beyond apredetermined value an easy-to-rupture portion formed on the upper ventplate ruptures thus releasing the gas inside the battery case tooutside, and the rupture pressure has been set according to the methodof setting rupture pressure as claimed in claim
 3. 16. Theexplosion-proof non-aqueous electrolyte secondary battery of claim 6,wherein the caulking withstand pressure of the sealing part has been setat least 10 kgf/cm² higher than the upper limit of the rupture pressureof the easy-to-rupture portion formed on the bottom surface of thebattery case.
 17. The explosion-proof non-aqueous electrolyte secondarybattery of claim 10, wherein the caulking withstand pressure of thesealing part has been set 10 kgf/cm² higher than the upper limit of therupture pressure of the easy-to-rupture portion formed on the bottomsurface of the battery case.